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Chloromethane

What is Chloromethane?

Chloromethane is a chlorinated volatile organic compound (VOC) also known as methyl chloride. Unlike many chlorinated solvents, chloromethane exists primarily as a gas at room temperature because of its very low boiling point.

Chloromethane occurs naturally from sources such as biomass burning and biological processes, but it is also manufactured for industrial use. Current applications include the production of silicones and other chemical intermediates, while historical uses included refrigeration and other industrial processes. ATSDR identifies chloromethane as a gas at room temperature and notes that it is used in the manufacture of adhesives, sealants, and silicones.

At contaminated sites, chloromethane may also form as a daughter product during the anaerobic degradation of more highly chlorinated methanes, including carbon tetrachloride, chloroform, and dichloromethane.

Chemical Properties

Chloromethane is highly volatile and exists as a gas under normal ambient conditions. As a result, it does not typically behave as a conventional LNAPL or DNAPL in the subsurface.

Property Value
Chemical Name Chloromethane
Common Name Methyl Chloride
Chemical Formula CH₃Cl
CAS Number 74-87-3
Molecular Weight 50.49 g/mol
Physical State at Room Temperature Gas
Boiling Point Approximately -24°C
Solubility in Water Approximately 5,000–7,000 mg/L

Chloromethane is relatively soluble in water but also has a high vapor pressure, allowing it to readily volatilize from soil and water. ATSDR describes chloromethane as highly soluble in water, highly mobile in soil, and readily volatilized because of its high vapor pressure.

Common Sources of Chloromethane

Chloromethane may be present due to:

  • Chemical manufacturing operations
  • Silicone manufacturing
  • Production of adhesives and sealants
  • Historical refrigerant use
  • Industrial chemical processing
  • Hazardous waste disposal
  • Releases from chemical storage or process equipment
  • Degradation of dichloromethane, chloroform, and carbon tetrachloride
  • Natural sources such as biomass combustion

At chlorinated solvent sites, chloromethane may occur together with carbon tetrachloride, chloroform, and dichloromethane as part of a sequential anaerobic degradation pathway.

Environmental Concerns

Chloromethane is of environmental concern because of its volatility, mobility, and potential to migrate through both groundwater and soil gas.

Under reducing conditions, carbon tetrachloride may undergo sequential dechlorination to chloroform, dichloromethane, and subsequently chloromethane. The appearance of chloromethane during remediation can therefore indicate continued transformation of more highly chlorinated parent compounds.

Because chloromethane is itself a contaminant, treatment performance should be evaluated based on the complete degradation pathway rather than the disappearance of parent compounds alone.

Chloromethane can biodegrade further under appropriate environmental conditions, although degradation rates depend on microbial populations, redox conditions, temperature, and other site-specific factors.

Behavior in Soil and Groundwater

Chloromethane contamination may exist as:

  • Dissolved contamination in groundwater
  • Vapor-phase contamination within the vadose zone
  • Contamination within soil pore water
  • A daughter product formed during degradation of more highly chlorinated methanes

Because chloromethane is highly mobile in soil and has limited tendency to bioaccumulate, it may migrate through the subsurface and reach groundwater. ATSDR notes that chloromethane's high soil mobility can allow it to migrate downward into groundwater.

Its high volatility also causes chloromethane to partition readily from contaminated soil and groundwater into soil gas. Vapor migration may therefore be an important consideration where contamination is present beneath or near occupied structures.

ATSDR estimates that chloromethane can persist in groundwater considerably longer than in surface environments, where volatilization may remove it more rapidly.

Human Health Effects

Exposure to elevated concentrations of chloromethane may affect the:

  • Central nervous system
  • Liver
  • Kidneys
  • Cardiovascular system
  • Reproductive system

Neurological effects are among the most sensitive health endpoints associated with chloromethane exposure. Human case reports involving elevated inhalation exposures have described headache, fatigue, dizziness, impaired coordination, slurred speech, and, at very high concentrations, coma and persistent neurological effects.

Liver effects have also been reported in human and animal studies. Animal studies provide additional evidence of kidney, cardiovascular, reproductive, and developmental effects following sufficiently high exposures.

EPA and the International Agency for Research on Cancer have determined that available evidence is insufficient to classify chloromethane with respect to human carcinogenicity.

Regulatory Standards

Regulatory criteria vary by jurisdiction. The United States Environmental Protection Agency has not established a federal Maximum Contaminant Level (MCL) for chloromethane in drinking water. Chloromethane is not included among EPA's federally regulated primary drinking-water contaminants with an enforceable MCL.

Because no federal MCL exists, site evaluations typically rely on applicable state groundwater standards, risk-based screening levels, vapor intrusion criteria, or other health-based regulatory values.

Many states have established their own groundwater, soil, soil gas, vapor intrusion, and risk-based screening criteria for chloromethane.